1. Field of the Invention
The present invention is directed to a solid poly-amphiphilic polymer. The polymer may be (1) a continuous film (a) which is strengthened sufficiently by cross-linking to be used alone and/or supported on a frame, (b) overlaid and/or cast on a porous hydrophobic support or (2) introduced into the pores of a microporous hydrophobic membrane. It is understood that the pores in either the porous hydrophobic support or microporous hydrophobic membrane lead from one surface of the support or membrane to the other.
The present invention is also a process for selectively removing a dissolved species, particularly polar organic compounds, (solute or target compound), which species may be a liquid or gas, from a gaseous stream or from an aqueous solution comprising contacting said gaseous stream or aqueous solution having the dissolved species and an aqueous stripping solution with opposite sides or surfaces of the solid poly-amphiphilic polymer alone, the polymer on a frame, the polymer on a porous hydrophobic support, or the polymer within the micropores of a hydrophobic membrane. In other words, the separation process of the present invention may be applied to a gas-gas, gas-liquid, or liquid-liquid separation.
2. Description of Related Art
Microporous membranes are well known. These membranes are fabricated from organic polymers as thin films or hollow fibers with continuous networks of interconnected pores leading from one surface to the other. The rate at which solvent, ions, monomer and polymer molecules, and other small particles pass through the pores depends not only on pore size but also on mutual attractions and repulsions between the membrane material and the materials either on the membrane in the pores or the membrane.
These membranes have been used for the separation of very small particles, such as colloids and polymers, from each other or from the liquid in which they are suspended, as separators in rechargeable batteries, in blood oxygenators, wherein the membrane has a liquid in contact with one surface and a gas in contact with the other surface, in other biological and medical applications of microporous membranes such as blood dialysis, where waste products are removed from blood.
The known membranes are also used as supports for liquid membranes, wherein a liquid which is imbibed in the pores of the microporous membrane is the medium through which transport takes place. That is it has been discovered that polyalkylene glycols (polyalkylene oxides) and polypropylene glycols in particular have a strong affinity for phenolic and related compounds such as phenol, nitrophenol, nitroaniline, and the like, and are especially useful for removing such compounds from aqueous solutions. Partition coefficients for these compounds, defined as the quotient or ratio of the concentration of the compound in the polypropylene glycol phase to the concentration of the compound in the aqueous phase with which it is in contact under equilibrium conditions, range from about 150 to over 500. These high partition coefficients are thought to be a consequence of concomitant hydrogen bonding and hydrophobic interaction between the organic compound and the poly-amphiphilic polymeric liquid lodged in a microporous membrane. These liquid membranes imbibed in the pores of the microporous membrane are found in copending U.S. applications Ser. Nos. 07/854,945 and 07/854,945 filed Mar. 20, 1992.
Microporous membranes are made from organic polymers by a variety of known methods. Organic polymers that are currently used to make microporous membranes include cellulose esters, as for example cellulose acetate; polyvinyl chloride; polysulfones and other high temperature aromatic polymers; polytetrafluoroethylene; polyolefins, including polypropylene and polyethylene; polycarbonates; polystyrene; and nylons.
Methods are known for modifying such membranes which involve reactions of monomers or oligomers with other monomers that have highly reactive functional groups. This leads to polymerization or cross-linking. For example, U.S. Pat. No. 3,744,642 describes a reverse osmosis membrane that is made by the interfacial condensation of a diamine and a diacid chloride within a porous substrate made of paper, glass fibers, or polymeric fibers, yielding a composite polyamide membrane. U.S. Pat. Nos. 3,951,815, 4,039,440, and 4,337,154 all are directed to the synthesis of composite reverse osmosis membranes by the cross-linking of amine containing polymers within a porous substrate. However, although the polymerizations are carried out in porous substrates, the resulting membranes are not in general microporous nor do these polymerizations result in the poly-amphiphilic polymers of the present invention.
The above-described approach has been further extended to include polymerizations and cross-linking reactions in the pores of microporous membranes in order to entrap water soluble polymers within the pore networks, thus rendering hydrophobic membranes hydrophilic. U.S. Pat. No. 4,113,912 teaches that a fluorocarbon microporous membrane, such as polyvinylidene fluoride, can be made hydrophilic by filling the pores with an aqueous solution of a water-soluble polymer, as for example polyacrylic acid, polyacrylamide, or polyvinyl alcohol, and then subjecting the polymer-treated membrane to reagents and conditions that lead to insolubilization of the polymer, generally by cross-linking. European Patent Application 257,635 teaches, that hydrophobic membranes, with fluorocarbon membranes used as examples, can be rendered hydrophilic by filling the pores with an aqueous solution containing one or more hydrophilic polyfunctional amine- or hydroxy-containing monomers or polymers, such as water-soluble cellulose derivatives or polyvinyl alcohol, along with cross-linking agents and optional catalysts, surfactants and initiators. The solutions described above are formulated with the goals of improving penetration of the pores and also of inducing cross-linking to take place or causing the hydrophilic compound to chemically bind to the fluorocarbon substrate.
In U.S. Pat. No. 5,049,275 a process is described for modifying the properties of a microporous membrane wherein a polymerizable vinyl monomer and a polymerization initiator are incorporated into the pores of a microporous membrane, and then the vinyl monomer is polymerized so that the polymerized monomer is secured in the pores of the membrane. The polymerizable vinyl monomer consists of one or more monofunctional vinyl monomers and an optional multifunctional vinyl monomer which can act as a cross-linking agent. The method is disclosed as being useful for modifying hydrophobic microporous membranes with hydrophilic monomers, as occurs for example when microporous polypropylene membranes are modified by filling the pores with acrylic acid followed by free radical polymerization.
Cipriano et al., Journal of Membrane Science, 61 (1991) 65-72, describe using a cast polyurea polyurethane membrane or film for pervaporation of an ethanol/water mixture. The membrane is prepared by polymerizing trifunctional isocyanate capped prepolymer made by reacting toluene diisocyanate with a trifunctional alcohol containing repeating ethoxy or propoxy groups. The prepolymer polymerized via a urea intermediate formed in the presence of atmospheric water results in structural parameters including structure net holes. (See the Abstract of the paper noted in the starred footnote which Abstract is set out in the Journal: Abstracts of Papers of the American Chemical Society, 1988, V195).